1. Field of the Invention
The present invention relates to radio reception systems. More particularly, the present invention relates to a radio reception system by a communication system such as the PDMA (Path Division Multiple Access) and the CDMA (Code Division Multiple Access) that can remove the interference signal component of another user from a reception signal.
2. Description of the Related Art
In the field of mobile communication systems such as portable telephones that have become extremely popular recently, various transmission channel allocation methods have been proposed to effectively use the frequencies. Some thereof are actually in practice.
FIGS. 27A–27C show the channel arrangement in various communication systems of FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and PDMA. The systems of FDMA, TDMA, and PDMA will be described briefly with reference to FIGS. 27A–27C.
Referring to FIG. 27A corresponding to the FDMA channel arrangement, the analog signals of users 1–4 are frequency-divided to be transmitted in radio waves of different frequencies f1–f4. The signals of respective users 1–4 are separated by frequency filters.
Referring to FIG. 27B corresponding to the TDMA channel arrangement, the digitized signals of respective users are time-divided and transmitted in radio waves of different frequencies f1–f4 at every constant period of time (time slot). The signals of respective users are separated by frequency filters and by time synchronization between a base station and each user's mobile terminal device.
Recently, the PDMA system has been proposed to improve the radio wave frequency usability to comply with the spread of portable telephones. In the PDMA system shown in FIG. 27C, one time slot of the same frequency is divided spatially to transmit data of a plurality of users. The signals of respective users in the PDMA system are separated by means of frequency filters, time synchronization between a base station and each user's mobile terminal device, and a signal extraction device such as an adaptive array.
FIG. 28 shows a conventional reception system of a base station for use in PDMA. Four antennas 3–6 are provided to distinguish between user 1 and user 2. The outputs of respective antennas are applied to a frequency conversion circuit 7 to be frequency-converted by a corresponding local oscillation signal Lo respectively and then converted into digital signals by an A/D converter 8. The signals are provided to a DSP (Digital Signal Processor) 10.
DSP 10 includes adaptive arrays 11 and 12, a reception signal vector calculator 13, a memory 14, a correlation value calculator 15, and a channel allocation apparatus 16. Each of adaptive arrays 11 and 12 extracts only a particular user signal from the reception signals output from A/D converter 8. Each adaptive array extracts the user signal specified by channel allocation apparatus 16 that will be described afterwards, depending on the weight vector calculation method such as the method using the preamble included in the time slot or the method taking advantage of the feature of the envelope line of a modulation signal being constant.
Reception signal vector calculator 13 receives the reception signals from A/D converter 8 and the output signals of adaptive arrays 11 and 12 to calculate reception signal vectors corresponding to all the users. The calculated reception signal vectors are stored in memory 14. Channel allocation apparatus 16 specifies two users with respect to memory 14 and correlation value calculator 15. Correlation value calculator 15 calculates the cross correlation value of the reception signal vectors of the two specified users out of the reception signal vectors stored in memory 14. Channel allocation apparatus 16 receives the calculated cross correlation value of the reception signal vectors of the two users. When the cross correlation value is equal to or smaller than a constant value, those two users are connected in path multiplex at the same time slot.
Adaptive arrays 11 and 12 shown in FIG. 28 extract the signals of corresponding users 1 and 2, respectively. When a user 3, for example, in addition to users 1 and 2, transmits a signal from a direction identical to that of user 1, a signal which is a mixture of the signals of users 1 and 3 will be output from adaptive array 11. However, the conventional adaptive array 11 cannot separate the signals of users 1 and 3. Therefore, it was not possible to extract only the signal of user 1.